The present invention relates to heat exchangers and specifically those which directly extract heat from high temperature media streams and transfer this heat to a heat sensitive working fluid and/or heat exchangers which combine a plurality of separate heat exchange zones within a single physical package.
Analyses of mobile waste heat recovery systems (WHRS) which extract energy from ICEs suggest that using a medium other than water, such as a refrigerant, is advantageous for a Rankine-cycle WHRS operating from heat sources lower than 650 C. Known issues with using water as a rankine cycle working fluid include: the potential for damage to the turbine and other parts of the WHRS flow path due to the corrosive nature of high temperature and pressure steam; and getting a high enough pressure ratio across the turbine. While this is feasible in a typical stationary steam power plant, which may run the working fluid up to a temperature of 600 C at 30 MPa, these conditions are difficult to achieve in a mobile application.
Refrigerant use, however, comes with a challenge—above fairly moderate temperatures (˜250 C for R245fa) the fluid is susceptible to permanent and irrevocable damage. A safe solution would be to use a pair of intermediate heat exchangers and a heat transfer fluid that could run at temperatures closer to the ICE exhaust gas temperature. This solution would add bulk, cost, and weight to the system.
A single stage heat exchanger without such an intermediate heat transfer solution would have opposing surfaces exposed to 560 C on one side and less than 250 C on the opposite side. Since heat transfer characteristics are inversely proportional to the thickness of the material between the fluids, one would want to minimize the thickness of the material. The thinner the sheet of material, the less surface area is required to transfer the heat, which leads to lower pressure drop, lower cost and reduced weight. However, thinner sheets also have a significant downside—internal stresses will be quite high due to the thermal stresses caused by the opposing surface temperatures and corresponding thermal expansion and strains. Finding a way to minimize the temperature differential on opposing sides of the sheet will provide the basis for the development of an efficient, low-cost, light-weight heat exchanger which is the core component of a WHRS.
Another challenge faced by mobile WHRS is the need to package the system compactly. Such systems typically comprise condensers, pumps, turbines, and heat exchangers. For a system which extracts heat from a plurality of sources, the heat exchangers can be the most volumetrically expensive system components. The reason for this is that in the existing art, each heat exchanger is a separate component, requiring its own mounting hardware, insulation, accessible inlets and outlets, fittings, insulated pipes, etc.